Antenna diversity is a well-known method for improving the performance of RF communications devices in a multipath propagation environment. In antenna diversity, two or more antennas operating at the same frequency band are used to receive the same information over independently fading radio channels. When the signal of one channel fades, the receiver can rely on the one or more other antennas to offer a better signal level. Ideally, the two or more antennas are positioned to provide uncorrelated signals. These signals are then combined according to one of the diversity techniques, such as switched diversity, selection diversity, equal gain and maximal ratio combining. It is also possible to use various interference rejection combining and interference suppression techniques. In general, diversity solutions can reduce the effects of fading and interference at the expense of increased complexity. Nevertheless, diversity can provide, for example, better telephone call quality, improved data rates and increased network capacity without the use of extra frequency spectrum. When implemented in mobile terminals, the benefits of antenna diversity can be achieved without investments in the network infra-structure.
Because of the small volume available for a mobile terminal antenna, it is challenging to design compact antennas that operate efficiently at multiple communication system bands, such as GSM850/(W) CDMA850 (824-894 MHz), GSM900 (880-960 MHz), GSM1800 (1710-1880 MHz), GSM1900/(W) CDMA (1850-1990 MHz) and UMTS (1920-2170 MHz). The designing task becomes even more challenging when additional diversity antennas operating at one or more of those system bands must be included in the same small volume in a mobile phone. In the talk position, one side of a mobile phone is typically covered by the user's head, while the other side is mostly covered by the user's hand. Thus, only a relatively small area and volume is available for the internal antenna system. In order to avoid being covered by the lossy tissues of the user's head and hand, all antennas should be placed within the available small area and volume, typically at the top section of the mobile phone. This leads to small electrical separation between the antennas. Generally, it can be difficult to achieve low correlation between closely spaced antennas. Typically, closely spaced antennas operating at the same frequency bands also couple strongly to each other. The coupling between antennas operating at the same frequency band generally reduces their efficiency. Consequently, the improvement that can be obtained with antenna diversity in noise-limited environment is also adversely affected.
It is thus advantageous and desirable to provide a compact multimode, multiband antenna system wherein a diversity antenna element is used for diversity reception or transmission or both (MIMO—multiple input multiple output).